The invention relates to a luminescent screen provided with a luminescent layer which is applied to a carrier and comprises a luminescent terbium-activated silicate having a cuspidine crystal structure. The invention further relates to a low-pressure mercury vapour discharge lamp provided with such a luminescent screen.
From the article in Philips Res. Repts. 1967, 22, 481-504, luminescent terbium-activated silicates are known, whose fundamental lattice corresponds to the formula Y.sub.2 Ca.sub.2 Si.sub.2 O.sub.9. This silicate and also the isomorphous compound Y.sub.4 Al.sub.2 O.sub.9, of which the terbium activation is also described, have the monoclinic crystal structure of the mineral cuspidine (Ca.sub.4 Si.sub.2 O.sub.7 F.sub.2). Terbium-activated silicates having a fundamental lattice of the type Y.sub.2 SiO.sub.5 and Y.sub.2 Si.sub.2 O.sub.7 are further known, for example, from U.S. Pat. Nos. 3,523,091 and 3,758,413.
The element terbium is a frequently used activator for luminescent materials because in many crystal lattices it gives rise to a very luminescence, the characteristic green Tb.sup.3+ emission being emitted. Upon excitation by ultraviolet radiation, however, it is a condition for obtaining an efficient luminescence that the luminescent material is excited by radiation having a wavelength lying in the maximum or very close to the maximum of the excitation spectrum of the material. An important application of such materials is found in low-pressure mercury vapour discharge lamps. In such lamps, mainly ultraviolet radiation having a wavelegth of about 54 nm is produced. A great disadvantage of many Tb-activated materials is that the maximum of the excitation band of the terbium is found at wave-lengths comparatively far from 254 nm. In those cases, an efficient luminescence can be obtained only if the excitation energy is first absorbed in a second activator, whereupon this energy is transferred to the terbium. During this absorption and transfer, of course losses may occur.
The invention has for its object to provide luminescent screens comprising new luminescent materials which are activated by terbium and which in practical applications can be directly excited in the terbium.